The Insight offers a combined mpg of 41, and an MSRP of $19,800. The Prius goes 50 mpg and has an MSRP of $22,800. So $3,000 more for nine more miles per gallon. That means if you drive 15,000 miles per year, you would need 300 gallons to fuel the Prius that distance, and about 366 to power the Insight. If gas is $4 a gallon, that means you'd be paying $264 dollars per year more to gas up the Insight compared to the Prius. That would take 11 years for the prices to even out with manufacturer suggested retail prices.
Gas will be going up over $4 a gallon nationally, and has already done so in California. So that time span will probably be much shorter. There are other things to consider for long-term ownership of these cars -- maintenance and repairs, durability and reliability, how long the battery lasts and what it costs to replace it, etc. So the Insight seems to make dollars and sense. But, do you like it enough? If you're really into the Prius, it would be a tough choice.